The invention relates to low molar mass aliphatic polyester polyols, to their preparation and to their use in coating compositions.
Present-day ready-to-spray clearcoat materials and pigmented topcoat materials, and also primer-surfacer materials, are relatively low in solvents, i.e., they contain fewer volatile organic compounds (VOCs) which are emitted into the atmosphere in the course of the application and drying of said coating materials. These more environmentally friendly coating compositions comprise low molar mass binders and a curing agent. In the case of a two-component (2K) coating composition curing agents are used which react even at low temperature with the functional groups of the binder to form a crosslinked film (usually polyfunctional isocyanates); in the case of one-component (1K) coating compositions, curing agents are used which react only at increased temperature (usually, for example, melamine resins).
In many cases, such binders are hydroxy-functional polyesters or polyacrylates, and also acrylic-modified polyesters or polyester-modified polyacrylates, or mixtures of these resins with one another, or resins of this kind to which reacted diluents have been added.
Reactive diluents are organic chemicals which, in the form of a mixture with the binder, reduce its viscosity, but which are able to react with the respective curing agent. In the case of 2K curing they are, for example, sterically bulky amine compounds (e.g., aspartic esters), blocked amine compounds (e.g., ketimines, aldimines) or blocked xcex2-hydroxyamine compounds (e.g., oxazolidines). Disadvantages of all of these compounds are their intrinsic yellow to brown color, the high yellowing propensity of coating materials comprising them, after application, the short pot life (about 1.5 h, whereas the requirement is for about 8 h, i.e., one working day), and the absence of application reliability under different weathering conditions in practice (e.g., unmasking of the amine compounds by exposure to atmospheric moisture). The pot life (paint processing time) is defined as the time within which the initial viscosity of the ready-to-apply coating material doubles. Accordingly, such compounds are unsuitable as additives for the abovementioned high performance coating systems.
It is also known that the drying rate of the so-called high solids coating materials (with a high mass fraction of solids) may be accelerated by means of external catalysis (for example, with dibutyltin dilaurate, zinc octoate, triethylenediamine, diethylethanolamine, volatile acids, etc.), although this also leads to a severe curtailment of the pot life or paint processing time. Moreover, attempts are made to improve the reactivity with isocyanate curing agents by modifying the binder, by altering the polarity of the polymer framework in the case of acrylate resins (e.g., by introducing carboxyl groups) (cf. EP-A 0 680 977).
Low molar mass polyester resins known to date, with rapid initial drying, contain special, expensive, sterically hindered units and have an acid number of from 5 to 35 mg/g (DE-A 198 09 461) but nevertheless have longer drying times until a dust-dry or tack-free state is achieved than the known systems containing the customary binders of relatively high molar mass. Moreover, coating materials containing these low molar mass polyesters have only a short pot life (about 3 h) and a surface quality (leveling) which is in need of improvement. There is therefore a desire for inexpensive low molar mass polyester resins without these special units and with more rapid drying, a long pot life, and a good surface without a propensity to yellow after application.
It has surprisingly been found that by using electron-rich heterocyclic functional units containing hydroxyl and/or epoxide groups, and using trifunctional hydroxy compounds additionally to difunctional hydroxy compounds, it is possible for the first time to obtain new low molar mass aliphatic OH-functional polyester resins which exhibit accelerated drying on isocyanate curing while nevertheless showing a sufficiently long pot life (from 6 to 8 h) and good leveling of the paint film and little propensity to yellow after application.
The electron-rich heterocyclic OH-functional and/or epoxy-functional units suitable for the invention are at least difunctional compounds which possess at least one cyclic structure and contain at least, in addition to carbon, hydrogen and oxygen, a heteroatom such as nitrogen, sulfur or phosphorus. This heteroatom is preferably in the vicinity of, and in particular positioned alpha to, a carbonyl function. A particularly suitable heteroatom is nitrogen. Examples of OH-functional monomer units of this kind are 4,5-dihydroxy-N,Nxe2x80x2-dimethylolethyleneurea or trishydroxyethyl isocyanurate (1,3,5-tris (2-hydroxyethyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione). An example of an epoxy-functional unit is triglycidyl isocyanurate.
The invention accordingly provides low molar mass polyester polyols having a weight-average molar mass Mm of up to 3500 g/mol, having hydroxyl numbers of from 80 to 280 mg/g and acid numbers of from 5 to 40 mg/g, containing the following mole fractions (in %=mol/100 mol) of structural units derived from
a) from 1 to 18%, preferably from 3 to 15%, with particular preference from 5 to 12%, of aliphatic monocyclic or polycyclic polyhydroxy or polyepoxy compounds A containing at least two hydroxyl and/or epoxy groups and at least one heteroatom, preferably a nitrogen atom, which is preferably positioned alpha to a carbonyl group,
b) from 1 to 30%, preferably from 3 to 25%, with particular preference from 5 to 20%, of aliphatic acyclic or cyclic polyhydroxy compounds B containing three or more hydroxyl groups per molecule,
c) from 15 to 50%, preferably from 20 to 45%, with particular preference form 25 to 40%, of linear or branched aliphatic dihydroxy compounds C,
d) from 25 to 60%, preferably from 30 to 55%, with particular preference from 35 to 50%, of aliphatic cyclic polycarboxylic acids D
and
e) from 0 to 20%, preferably from 1 to 15%, with particular preference from 2 to 10%, of polyfunctional compounds E selected from aliphatic linear and branched dicarboxylic acids, aromatic dicarboxylic acids and polycarboxylic acids containing three or more carboxyl groups per molecule, and also
f) from 0 to 15%, preferably from 1 to 12%, with particular preference from 2 to 10%, of monofunctional units F selected from monocarboxylic acids, monoalcohols and monoepoxides,
the ratio of the sum of the amounts of substance of hydroxyl groups and epoxide groups of the components A and B and the amount of substance of hydroxyl groups of the component C being at least 1 or greater than 1, and the mole fractions indicated respectively under a), b), c), d), e) and f) adding up to 100%.
The acid number is defined in accordance with DIN EN ISO 3682 as the ratio of that mass MKOH of potassium hydroxide which is required to neutralize a sample for analysis to the mass mB of the sample (mass of the solids in the sample in the case of solutions or dispersions); its customary unit is mg/g. The hydroxyl number is defined in accordance with DIN EN ISO 4629 as the ratio of that mass mKOH of potassium hydroxide which has exactly the same number of hydroxyl groups as a sample for analysis to the mass mB of said sample (mass of the solids in the sample in the case of solutions or dispersions); its customary unit is mg/g.
The aliphatic monocyclic or polycyclic polyhydroxy or polyepoxy compounds A have preferably 5- or 6-membered rings in which at least one atom is other than carbon and is preferably a nitrogen atom. The carbonyl group preferably adjacent to the nitrogen atom is likewise preferably part of the ring. Particular preference is given to those compounds A having two or three nitrogen atoms in one ring. The hydroxyl groups are preferably present in methylol, hydroxyethyl or 2-hydroxypropyl groups. Examples of suitable compounds are N,Nxe2x80x2-dimethylol-2-imidazolidone, N,Nxe2x80x2-dimethylol-4,5-dihydroxy-2-imidazolidone, the reaction products of parabanic acid or 2-imidazolidone (ethyleneurea) with oxirane or methyloxirane having in each case two hydroxyl groups, and the reaction products of glycoluril (acetyleneurea) with formaldehyde, oxirane or methyloxirane, ranging up to the corresponding tetrahydroxymethyl, tetrahydroxyethyl or tetrahydroxypropyl glycolurils. A particularly preferred compound is the abovementioned trishydroxyethyl isocyanurate or the homologous trishydroxypropyl isocyanurate obtainable by reaction with methyloxirane. It is also possible to prepare mixed hydroxyalkyl derivatives by reacting mixtures of oxirane and methyloxirane with isocyanuric acid. The reaction of epichlorohydrin with isocyanuric acid produces triglycidyl isocyanurate, which is preferred as a polyepoxy compound. Other epoxy-functional compounds A may be prepared similarly by reacting epichlorohydrin with the aforementioned cyclic ureas, examples being diglycidylethyleneurea, diglycidylparabanic acid or tetraglycidylacetyleneurea. By reaction with mixtures of, for example, oxirane or methyloxirane with epichlorohydrin it is possible to prepare compounds A with mixed functionalities.
The aliphatic acyclic or cyclic polyhydroxy compounds B containing three or more hydroxyl groups per molecule have preferably from 3 to 20, more preferably from 3 to 12 carbon atoms and may also be linear or branched. Examples of suitable compounds are glycerol, trimethylolethane, trimethylolpropane, 1,2,6-trihydroxyhexane, 1,2,3-trihydroxyheptane, erythritol, pentaerythritol, sorbitol, xylitol, and mannitol, and also ditrimethylolethane, ditrimethylolpropane, diglycerol and dipentaerythritol. It is also possible to use reaction products of these compounds with oxirane or methyloxirane.
The linear or branched aliphatic dihydroxy compounds C have preferably from 2 to 20 carbon atoms, in particular from 2 to 9 carbon atoms. Preference is given to primary, and particular preference to diprimary, hydroxy compounds. Examples of suitable compounds are glycol, 1,2- and 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, 2-methyl-1,3-propanediol, 2-methyl-2-butyl-1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, pinacol, bishydroxymethylheptane, and 2,2,4- and also 2,4,4-trimethyl-1,6-hexanediol. Particular preference is given to the use of mixtures of linear and branched dihydroxy compounds.
The aliphatic cyclic polycarboxylic acids D have from 6 to 12 carbon atoms and are preferably selected from the cyclic dicarboxylic acids 1,2- and 1,4-cyclohexanedicarboxylic acid and the dicyclic dicarboxylic acids cis-5-norbornene-endo-2,3-dicarboxylic acid and methyl-5-norbornene-2,3-dicarboxylic acid and also the cyclic tricarboxylic acid 1,3,5-cyclohexanetricarboxylic acid.
As polyfunctional compounds E it is possible to use aliphatic linear and branched dicarboxylic acids, aromatic dicarboxylic acids and polycarboxylic acids containing three or more carboxyl groups per molecule. Preference is given to aliphatic dicarboxylic acids having from 2 to 40 carbon atoms such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebatic acid, branched aliphatic dicarboxylic acids such as dimethylsuccinic acid, butylmalonic acid, diethylmalonic acid, dimethylglutaric acid and methyladipic acid, and also the hydrogenated fatty acid dimers and mixtures of these compounds; aromatic dicarboxylic and polycarboxylic acids such as phthalic acid, isophthalic and terephthalic acid, 2,3-, 1,4- and 2,6-naphthalenedicarboxylic acid, 4,4xe2x80x2-biphenyldicarboxylic acid, 4,4xe2x80x2-sulfonyldibenzoic acid, 4,4xe2x80x2-diphenyl ether dicarboxylic acid, 4,4xe2x80x2-benzophenonedicarboxylic acid, and also trimellitic acid, trimesic acid, pyromellitic acid, and benzophenonetetracarboxylic acid.
As monofunctional units F it is possible to use aliphatic or aromatic monocarboxylic acids such as acetic acid, ethylhexanoic acid, isononanoic acid or benzoic acid or aliphatic monoalcohols having from 4 to 20 carbon atoms such as ethanol, n-butanol, tert-butanol, amyl alcohol, 2-ethylhexanol, isononyl alcohol or isotridecyl alcohol. Monoepoxides such as the glycidyl esters of branched monocarboxylic acids may also be used.
Instead of the acids and hydroxy compounds, the low molar mass polyester polyols of the invention may also be synthesized using ester-forming derivatives of these compounds, such as esters of the acids with lower aliphatic alcohols (having from 1 to 4 carbon atoms, linear or branched, primary, secondary or tertiary alcohols), preferably methyl esters, acid anhydrides or acid halides, and also esters of the hydroxy compounds with volatile organic acids, such as acetates or propionates, for example.
The low molar mass polyester polyols of the invention are prepared in the conventional manner by mixing the reactants and condensing them together at elevated temperature. The condensation reaction may be accelerated conventionally by removing the water (or other condensates when using derivatives of the acids and alcohols employed) formed during the reaction, by distillation under reduced pressure. The polycondensation may also be conducted in solvents which form an azeotrope with water; the polycondensation may be performed with particular efficiency by distillation, separation of the water, and recycling of the solvent. The polycondensation may also be conducted in the presence of catalysts.
For the present invention it is preferred to prevent discolorations of the polyester polyol produced by adding reducing agents during the condensation. For this purpose it is possible, for example, to use phosphites or compounds of hypophosphorous acid; hydrogen peroxide may also be added.
The low molar mass polyester polyols of the invention may be prepared batchwise (in a batch process) or continuously. Multistage processes at atmospheric pressure and with increased pressure are also possible.
The extremely low molar mass, OH- and COOH-functional polyester polyols of the invention may be dissolved to a selectable extent using freely selectable solvents or solvent mixtures.
The hydroxyl- and carboxyl-containing, low molar mass polyester polyols prepared in accordance with the invention may further be chemically or physically modified, for example, by reaction with isocyanate compounds or compounds containing oxirane groups. On the low molar mass polyester polyol of the invention, the reaction with isocyanate compounds leads to urethane groups. The reaction with the oxirane compounds leads to additional (secondary) OH groups.
It is also possible at the same time to prepare low molar mass urea derivatives, which in the coatings industry lead to what are known as sag-controlled resins. For this purpose, for example, the low molar mass polyester polyol is introduced as a mixture with monoamines or polyamines, and suitable monofunctional or polyfunctional isocyanates are added. In this context it is advantageous to adopt a procedure whereby polyfunctional isocyanates, when using monoamines, and monofunctional or partly blocked polyfunctional isocyanates, when using polyamines, are employed. Suitable amines are primary aliphatic linear or branched or cyclic amines having from 12 to 18 carbon atoms such as butylamine, hexylamine, 2-ethylhexylamine, dodecylamine and stearylamine, aliphatic, alicyclic and aromatic diamines such as ethylenediamine, 1,4-diaminobutane and 1,6-diaminohexane; 1,3-bisaminomethylcyclohexane and 2,2-bis(4-aminocyclohexyl)propane, and also meta-xylylenediamine. Suitable isocyanates are the polyfunctional isocyanates known from paint chemistry such as trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate (HDI), 1,2-propylene diisocyanate, ethylethylene diisocyanate, 2,3-dimethylethylene diisocyanate, 1-methyltrimethylene diisocyanate, 1,3-cyclopentylene diisocyanate, 1,4-cyclohexylene diisocyanate, 1,2-cyclohexylene diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-tolylene diisocyanate (TDI), 2,6-tolylene diisocyanate (TDI), 4,4xe2x80x2-biphenylene diisocyanate, 1,5-naphthylene diisocyanate, 1,4-naphthylene diisocyanate, 1-isocyanatomethyl-5-isocyanato-1,3,3-trimethylcyclohexane (IPDI), bis(4-isocyanatocyclohexyl)methane (HMDI), 4,4xe2x80x2-diisocyanatodiphenyl ether, 2,3-bis(8-isocyanatooctyl)-4-octyl-5-hexylcyclohexene, the isomeric trimethylhexamethylene diisocyanates, and tetramethylxylylene diisocyanate (TMXDI). Mixtures of such diisocyanates or polyisocyanates may likewise be used. Suitable monofunctional isocyanates include aliphatic, cycloaliphatic and aromatic isocyanates having up to 25 carbon atoms. Examples are methyl isocyanate, ethyl isocyanate, propyl isocyanate, butyl isocyanate, stearyl isocyanate, phenyl isocyanate, naphthyl isocyanate, tolyl isocyanate, cyclohexyl isocyanate, tert-butyl isocyanate, and 2-ethylhexyl isocyanate. Examples of suitable partly blocked isocyanates are polyfunctional isocyanates (e.g., the difunctional or polyfunctional ones mentioned above) which have been reacted with, for example, monoalcohols. The advantage of such sag-controlled resins is that even relatively high film thicknesses may be produced in a single application without the formation of the defects known as runs or noses, caused by the paint running off from surfaces inclined in relation to the horizontal. Especially in the context of the application of high yield coating materials such as primer-surfacers, pigmented topcoat materials and special clearcoat materials, these problems may be avoided by using binders formulated or modified in this way. Binders which comprise the low molar mass polyester polyols of the invention having this sag-controlled modification possess not only this higher running limit but also good transparency, and make it possible to formulate coating materials having a high solids fraction. In clearcoat materials, accordingly, it is possible to achieve a mass fraction of solids (measured on coating materials having a flow time of 21 seconds from the DIN 4 cup at 23xc2x0 C.) of 60%, with a running limit of 60 xcexcm, and the coatings obtained exhibit outstanding optical properties and good gloss.
Moreover, the low molar mass polyester polyols of the invention may have been esterified (modified) with phosphoric acid, at least to a partial extent.
Preferably, the polyester polyols may also be mixed with acrylate copolymers, especially those of low molar mass. Another possibility for modification is for the novel low molar mass polyester polyols to form the basis of (grafted-on) acrylate polymers, as described in EP-A 0 776 920 or in EP-A 0 896 991. In both cases, either the ester resin character (mass fraction of the acrylate component less than 50%) or the acrylate resin character (mass fraction of the acrylate component greater than 50%) predominate. Of course, a composition with equal fractions of ester resin and acrylate resin is also possible.
The invention therefore further provides mixtures comprising the low molar mass polyester polyols and a copolymer prepared separately or in the presence of said polyester polyol by means of free-radical polymerization, the monomer mixture on which the copolymer is based comprising
at least one olefinically unsaturated monomer G which is an alkyl ester of an aliphatic linear, branched or cyclic xcex1,xcex2-unsaturated monocarboxylic acid or an alkyl diester of an olefinically unsaturated aliphatic linear, branched or cyclic dicarboxylic acid having from 1 to 20, preferably from 2 to 12, carbon atoms in the linear, branched or cyclic alkyl radical and from 3 to 10, preferably from 4 to 7, carbon atoms in the acid radical of the ester,
also at least one hydroxyalkyl ester H of one of the monocarboxylic or dicarboxylic acids mentioned under G, the hydroxyalkyl radical being derived from an at least dihydric aliphatic linear, branched or cyclic alcohol having from 2 to 15, preferably from 3 to 8, carbon atoms,
at least one carboxylic acid I selected from the monocarboxylic acids and dicarboxylic acids mentioned under G, in unesterified form or, in the case of the dicarboxylic acids, in a form in which it is monoesterified with one of the alkyl radicals mentioned under G or one of the hydroxyalkyl radicals mentioned under H, and also, if desired
at least one further olefinically unsaturated monomer J selected from vinylaromatics such as styrene, xcex1-methylstyrene, vinyltoluene, chlorostyrene, vinyl esters of aliphatic linear, branched or cyclic monocarboxylic acids having from 2 to 20, preferably from 3 to 12, carbon atoms, vinyl halides such as vinyl chloride, vinylidene chloride, unsaturated nitriles such as acrylonitrile and methacrylonitrile, amides and/or diamides of the acids mentioned under I, vinyl ethers and allyl ethers of aliphatic linear, branched or cyclic alcohols havig from 1 to 18 carbon atoms, esters of glycidyl alcohol or methylglycidyl alcohol with olefinically unsaturated carboxylic acids, and olefinically unsaturated ketones having from 4 to 21 carbon atoms.
These mixtures may be prepared by adding the copolymer to the polyester polyol, in which case the mass ratio of the solids fractions of the polyester polyol and of the copolymer is from 1:9 to 9:1, preferably from 7:3 to 3:7. The two components are normally mixed by intimate mixing of the solutions of both components. Such mixtures are referred to as blends.
The mixtures may also be prepared in accordance with the invention preferably such that the monomer mixture on which the copolymer is based is polymerized in the presence of the polyester polyol, in which case the ratio of the mass of the solids fraction of the polyester polyol to the mass of the monomer mixture on which the copolymer is based is from 9:1 to 1:9, preferably from 7:3 to 3:7. Such a polymer is referred to as a partially grafted polymer.
An advantage when using such mixtures or partially grafted polymers to formulate coating materials arises out of the fact that, with comparable performance properties (hardness, drying rate), it is possible to tailor the mass fraction of solids (for a specified flow time) and the hydroxyl number; in the case of a relatively low hydroxyl number, the amount of the curing agent needed for the specified level of properties can be adapted and thus the crosslinking density can be varied for a specified hardness of the coating film.
The monomer mixture comprises in both cases preferably mass fractions of
g) from 25 to 80% of alkyl (meth)acrylates G whose alkyl radicals may be linear, branched or cyclic and which have from 1 to 15 carbon atoms,
h) from 1 to 35% of hydroxyalkyl (meth)acrylates H whose hydroxyalkyl radicals may be linear, branched or cyclic and which have from 2 to 20 carbon atoms,
i) from 0.5 to 20% of xcex1,xcex2-unsaturated carboxylic acids I, and
j) from 0 to 55% of compounds J selected from aromatic vinyl compounds, aliphatic vinyl esters and vinyl ethers, allyl ethers, vinyl halides, olefinically unsaturated ketones, esters of glycidyl alcohol or methyl glycidyl alcohol with olefinically unsaturated carboxylic acids, and nitrites of xcex1,xcex2-unsaturated carboxylic acids, the sum of the mass fractions of the components G to J necessarily being 100% and the components G to J preferably being selected such that polymerization of this mixture without further additions would result theoretically in an acrylate copolymer having an acid number of at least 5 mg/g and a hydroxyl number of from 30 to 190 mg/g. The polymerization takes place in the presence of known free-radical polymerization initiators and also, if desired, of a regulator.
The component G is preferably selected from esters of acrylic and methacrylic acid with methanol, ethanol, n- and iso-propanol, n-, sec-, iso- and tert-butanol, and also isobornyl and isofenchyl alcohol.
The component H is preferably selected from esters of acrylic and methacrylic acid with glycol, 1,2- and 1,3-propanediol and 1,4-butanediol.
The component I is preferably selected from acrylic and methacrylic acid.
The component J is preferably selected from styrene, the isomeric vinyltoluenes, and xcex1-methylstyrene.
In the context of the invention it is a further preferred procedure to conduct the polymerization of the components G to J in the presence of a cyclic compound K, it being possible for said compound K to react with the compounds used as component I and/or with the compounds used as component H to form a copolymerizable compound. Suitable compounds K are epoxides, especially glycidyl esters of alpha-branched aliphatic monocarboxylic acids having from 4 to 12 carbon atoms in the acid group, which react with the acids I to form an unsaturated hydroxy ester, lactones or lactams, which react with the acids I to form an unsaturated acid or with the hydroxyl-containing compounds H to form unsaturated hydroxy compounds or amines. Procedures of this kind are described in EP-A 0 027 931, in WO 90/03991, and in EP-A 0 635 523, 0 638 591, 0 680 977, 0 714 915, and 0 741 149, whose disclosure content insofar as it relates to these processes is incorporated herein by reference.
The low molar mass polyester polyols of the inventionxe2x80x94alone and in mixturesxe2x80x94are especially suitable for coatings applications in one- and two-component systems, especially for what are known as high solids systems, i.e., solventborne mixtures with a high mass fraction of solids.
Suitable solvents for the oligoester polyols of the invention and, respectively, for the mixtures comprising said polyols are, for example, aliphatic, cycloaliphatic and aromatic hydrocarbons, such as alkylbenzenes, e.g., xylene, toluene; esters, such as ethyl acetate, butyl acetate, acetates with longer alcohol radicals, butyl propionate, pentyl propionate, ethylene glycol monoethyl ether acetate, the corresponding methyl ether acetate and propylene glycol methyl ether acetate; ethers, such as ethylene glycol monoethyl, monomethyl or monobutyl ether; glycols; alcohols; ketones such as methyl isoamyl ketone, methyl isobutyl ketone; lactones, and mixtures of such solvents. As solvents, it is also possible to use reaction products of lactones with glycols or alcohols.
The present invention additionally provides coating compositions which comprise the low molar mass polyester polyols of the invention or their sag-controlled modifications, in a blend if desired with other organic polyhydroxy compounds or with reactive diluents (low molar mass compounds which, alone or else together with the low molar mass polyester polyols or other co-components, react with the curing agents used). Particularly suitable co-components are acrylate copolymers of the type described above. These high-solids coating compositions are employed in particular in the coating of metal sheets (especially in OEM automotive finishing and refinishing, and for general industrial applications, such as steel bridges, for example), in the coating of plastics and wood, and in the field of coating of textiles, leather, paper, and construction materials.
The low molar mass polyester polyols or their sag-controlled modifications, and mixtures comprising these low molar mass polyester polyols, may be cured cold or at elevated temperature in the presence of appropriate crosslinkers (curing agents).
Suitable curing components in these coating compositions include amino resins, polyisocyanates, or compounds containing anhydride groups, individually or in combination. The crosslinker is added in each case in an amount such that the ratio of the number of OH groups of the low molar mass polyester polyol (or of the mixtures comprising it) to the number of reactive groups of the crosslinker is between 0.3:1 and 3:1.
Amino resins suitable as curing components are preferably urea resins, melamine resins and/or benzoguanamine resins. These are preferably etherified products of the condensation of urea, melamine or, respectively, benzoguanamine with formaldehyde. Suitable mixtures lie in the range from 50:50 to 90:10 for the ratios of the masses of the low molar mass polyester polyols and of the crosslinkers, based in each case on the mass of the solid resin. Suitable phenolic resins and derivatives thereof may also be employed as curing agents. In the presence of acids, e.g., p-toluenesulfonic acid, these crosslinkers result in curing of the coating. Heat curing may be performed conventionally at temperatures from 85 to 200xc2x0 C. in from 10 to 30 minutes, for example.
For the curing of the products of the invention with crosslinking, polyisocyanates are suitable, especially at moderate temperatures or at room temperature. Suitable polyisocyanate components include in principle all aliphatic, cycloaliphatic or aromatic polyisocyanates known from polyurethane chemistry, individually or in mixtures. Examples of highly suitable polyisocyanates are low molar mass polyisocyanates such as hexamethylene diisocyanate, 2,2,4- and 2,4,4-trimethyl-1,6-hexamethylene diisocyanate, dodecamethylene diisocyanate, tetramethyl-p-xylylene diisocyanate, 1,4-diisocyanatocyclohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), 2,4xe2x80x2- and 4,41-diisocyanatodicyclohexylmethane, 2,4xe2x80x2- and 4,4xe2x80x2-diisocyanatodiphenylmethane, and also mixtures of these isomers with their higher homologues, as obtainable in conventional manner by phosgenating aniline/formaldehyde condensates; 2,4- and 2,6-diisocyanatotoluene, and any desired mixtures of such compounds.
It is preferred, however, to use derivatives of these simple polyisocyanates, as are customary in coatings technology. These include polyisocyanates containing, for example, biuret groups, uretdione groups, isocyanurate groups, urethane groups, carbodiimide groups or allophanate groups, as described, for example, in EP-A 0 470 461.
The particularly preferred modified polyisocyanates include N,Nxe2x80x2,Nxe2x80x3-tris(6-isocyanatohexyl)biuret and its mixtures with its higher homologues, and also N,Nxe2x80x2,Nxe2x80x3-tris(6-isocyanatohexyl) isocyanurate and its mixtures with its higher homologues containing more than one isocyanurate ring.
Further compounds suitable for curing at elevated temperature include blocked polyisocyanates, and also polycarboxylic acids and their anhydrides.
The low molar mass polyester polyols of the invention and the mixtures comprising these are especially suitable for preparing high-solids solventborne clearcoat and topcoat materials and also for primer-surfacers.
Coating compositions are prepared by mixing the polyester, polyols (or mixtures comprising these) with the curing agents mentioned-above.
It is also possible for other auxiliaries and additives to be present which are customary in coatings technology but have not yet been mentioned in coating compositions which are prepared with the low molar mass polyester polyols of the invention or mixtures comprising them. These include, in particular, catalysts, leveling agents, silicone oils, additives such as cellulose esters, especially cellulose acetobutyrate, plasticizers, such as phosphates and phthalates, pigments such as iron oxides, lead oxides, lead silicates, titanium dioxide, barium sulfate, zinc sulfide, phthalocyanine complexes, etc., and fillers such as talc, mica, kaolin, chalk, quartz flour, asbestos flour, slate flour, various silicas, silicates, etc., viscosity additives, dulling agents, UV absorbers and light stabilizers, antioxidants and/or peroxide scavengers, defoamers and/or wetting agents, active diluents, and the like.
The coating compositions may be applied to the respective substrate in accordance with known methods, for example, by brushing, dipping, flowcoating or by roller coating or knifecoating, but especially by spraying. They may be applied under hot conditions, and if appropriate may be brought into a ready-to-apply form by injecting supercritical solvents (e.g., CO2). Automotive, industrial, plastics, wood, construction-material and textile coating materials having excellent properties may be obtained with binders or binder mixtures prepared using the low molar mass polyester polyols of the invention. These binders may be used both for producing intermediate coats and for producing pigmented or unpigmented topcoats.
For this purpose the coating materials are generally cured following their application, within the temperature range from xe2x88x9220 to +100xc2x0 C., preferably from xe2x88x9210 to +80xc2x0 C.
In the examples which follow, as in the text which precedes them, all figures with the unit xe2x80x9c%xe2x80x9d are mass fractions (ratio of the mass of the substance in question to the mass of the mixture), unless specified otherwise. Parts are always mass fractions. Concentrations in xe2x80x9c%xe2x80x9d are mass fractions of the dissolved substance in the solution (mass of the dissolved substance, divided by the mass of the solution).